1. Field of the Invention
The invention relates to an automatic transmission mounted on a vehicle. More specifically, it relates to a shift control device for the automatic transmission having a frictional engagement element connected via a one-way clutch and a frictional engagement element for a coasting state.
2. Description of the Prior Art
In general, an automatic transmission has a frictional engagement element connected with a one-way clutch in series. When the frictional engagement element is released and the automatic transmission performs a down shift, the one-way clutch is synchronistically engaged automatically. During a coasting state, power can not be transmitted from the wheels to the engine because of interposition of the one-way clutch. Therefore, the automatic transmission also has a frictional engagement element which is not connected with a one-way clutch during the coasting state in order to enable engine braking. The frictional engagement element that is not connected in series with a one-way clutch is arranged in parallel with the frictional engagement element connected in series with a one-way clutch.
A conventional shift control device having the aforementioned two frictional engagement elements in parallel with each other is disclosed in Japanese published patent application laid-open No. 219949/88. FIG. 10(a) indicates the control of that device applied to the embodiment of the invention. A second frictional engagement element connected in series with a one-way clutch is made to be B2. A first frictional engagement element for a coasting state is made to be B1. Hydraulic pressures of the hydraulic servos for B2 and BI are made to be PB2 and PB1 individually, and adjusting signals for PB2 and PB1 are made to be PB2C and PB1C individually. After piston stroke control in the hydraulic servos is performed to bring the functional engagement element to a state just before a torque is transmitted, the hydraulic pressure PB2 for the hydraulic servo connected to the second frictional engagement element B2 is increased to perform engagement control. Then the hydraulic pressure PB1 for the first frictional engagement element B1 for a coasting state is controlled to an engaging state. At that time, an input rotation speed N.sub.T is detected, the oil pressure for the second frictional engagement element B2 is controlled, and shift end, that is a synchronized rotation speed after the shift, is detected.
In the aforementioned shift control, in the case of a high load state, for example in the case of power on, the hydraulic pressure PB2 for the second frictional engagement element can be controlled properly with a feedback control based on the input rotation speed N.sub.T as shown in FIG. 10(a). But in the case of a low load state, for example in the case of power off, it is difficult to control a low torque properly because a rate of an increase of a torque capacity against an increase of the hydraulic pressure PB2 is large. That is because the hydraulic pressure PB2 has a large capacity in order to be able to activate a vehicle driving state. Therefore, the input rotation speed N.sub.T is decreased too much resulting in a large deviation from the synchronized rotation speed because the second frictional engagement element is connected via the one-way clutch. When the frictional engagement element for the coasting state is engaged and the input rotation speed N.sub.T becomes the synchronized rotation speed, a shift shock is caused.